Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

A concentrator photovoltaic device includes: an optical concentrator; a
solar battery cell; a homogenizer; a sealant; and a light transmission
preventing layer. The homogenizer has a trapezoidal shape in which a
sectional area at the optical concentrator side is larger than a
sectional area at the solar battery cell side, a relationship of
nh>nf>nt is satisfied among a refractive index
nh of the homogenizer, a refractive index nf of the sealant,
and a refractive index nt of the light transmission preventing
layer, a thickness (H) of the light transmission preventing layer is
equal to or larger than 0.1 mm and equal to or smaller than 1.2 mm, and a
relationship of 0.5≦b/a<1.0 is satisfied between a height (b)
of the light transmission preventing layer and a height (a) of the
sealant at a position where the light transmission preventing layer is
formed.

Claims:

1. A concentrator photovoltaic device comprising: an optical concentrator
for concentrating sunlight; a solar battery cell; a homogenizer which is
provided above the solar battery cell such that a bottom surface thereof
faces the solar battery cell and which guides sunlight concentrated by
the optical concentrator to the solar battery cell; a sealant which
covers a side surface of a lower portion of the homogenizer and the solar
battery cell; and a light transmission preventing layer provided between
the sealant and the homogenizer, wherein the homogenizer has a
trapezoidal shape in which a sectional area at the optical concentrator
side is larger than a sectional area at the solar battery cell side, a
relationship of nt>nf>nt is satisfied among a
refractive index n of the homogenizer, a refractive index nf of the
sealant, and a refractive index nt of the light transmission
preventing layer, a thickness (H) of the light transmission preventing
layer is equal to or larger than 0.1 mm and equal to or smaller than 1.2
mm, and a relationship of 0.5.ltoreq.b/a<1.0 is satisfied between a
height (b) of the light transmission preventing layer and a height (a) of
the sealant at a position where the light transmission preventing layer
is formed.

2. The concentrator photovoltaic device according to claim 1, wherein the
thickness (H) of the light transmission preventing layer is equal to or
larger than 0.2 mm and equal to or smaller than 1.0 mm.

3. The concentrator photovoltaic device according to claim 1, wherein a
relationship of 0.6.ltoreq.b/a≦0.98 is satisfied between the
height (b) of the light transmission preventing layer and the height (a)
of the sealant at the position where the light transmission preventing
layer is formed.

Description:

CROSS REFERENCE TO RELATED APPLICATION

[0001] The present application claims priority from Japanese Patent
Application No. 2010-117417, which was filed on May 21, 2010, the
disclosure of which is herein incorporatedby reference in its entirety.

TECHNICAL FIELD

[0002] The present invention relates to a concentrator photovoltaic device
and more specifically, to a concentrator photovoltaic device which
generates power by homogenizing high-energy sunlight concentrated by an
optical concentrator (primary optical system) with a homogenizer
(secondary optical system) and emitting the homogenized sunlight to a
solar battery cell.

BACKGROUND

[0003] Solar power generators are categorized roughly into a
non-concentrator photovoltaic device which emits sunlight to a solar
battery cell as it is and a concentrator photovoltaic device which emits
sunlight, which is concentrated by an optical concentrator, to a solar
battery cell. In the case of the concentrator photovoltaic device, a
solar battery cell can be made small. Accordingly, even if an expensive
cell with good conversion efficiency is used, the effect on the cost of
manufacturing electric power is slight. Therefore, the concentrator
photovoltaic device is advantageous in that inexpensive electric power
can be efficiently generated.

[0004] Light concentrated by the optical concentrator has a high intensity
at its centre and a low intensity at its periphery. Even if such light is
directly emitted to a solar battery cell, it is not possible to obtain
high power generation efficiency. Therefore, in the concentrator
photovoltaic device, a columnar or trapezoidal (tapered) optical member
called a homogenizer is usually provided immediately above the solar
battery cell. The homogenizer serves to homogenize light energy by
repeatedly performing total reflection of high-energy sunlight, which is
concentrated by the optical concentrator, on the side surface. Generally,
glass with high light transmittance is used for the homogenizer. In
particular, sodium containing glass, such as borosilicate glass and
silicate glass, is used for the homogenizer since it is an all-purpose
and cheap material and can be easily processed.

[0005] In addition, a solar battery cell is easily deteriorated by
moisture. For example, group III-V compound semiconductor represented as
InGaP/InGaAs/Ge is active compared with a crystalline silicon based
semiconductor. Therefore, a solar battery cell using the group III-V
compound semiconductor is noticeably deteriorated by moisture. In
addition, an antireflection film is generally provided on the surface of
a solar battery cell. However, the antireflection film may be altered if
the antireflection film comes in contact with moisture. In addition, the
sodium contained in the homogenizer may dissolve in condensed water and
sodium ions may reach the solar battery cell through moisture. The sodium
ions which reach the solar battery cell have accumulated a negative
electric potential of the surface of the solar battery cell, resulting in
a reduction of power generation efficiency. Therefore, in order to
improve the durability of the concentrator photovoltaic device, it is
necessary to protect the solar battery cell against moisture.

[0006] Various proposals have been made regarding a method of protecting
such a solar battery cell from moisture.

[0007] For example, Patent Document 1 discloses a concentrating type solar
power generating unit that uses a material containing 10% by weight or
more of fluorinated silicon resin as a sealing resin (sealant) which
covers the columnar optical member (homogenizer) and the solar battery
cell facing the bottom surface.

[0008] Patent Document 1 also discloses the following points.

[0009] (a) If a material containing 10% by weight or more of fluorinated
silicon resin is used as a sealing resin, permeation of vapor is
suppressed due to the low vapor transmission property of the fluorinated
silicon resin.

[0010] (b) A thin film, which is formed of fluororesin (refractive index:
1.34) with a thickness of about 10 nm to 20 nm and which functions as a
protective member or a water repellent film, may be formed on the side
surface of the homogenizer.

[0011] In addition, Patent Document 2 discloses a concentrator
photovoltaic device in which transparent resin is provided between the
bottom surface of the columnar optical member and the solar battery cell
and which includes a light shielding member for blocking sunlight from
the transparent resin.

[0012] Patent Document 2 also discloses the following points.

[0013] (a) Since photodegradation of the transparent resin is suppressed
by the light shielding member, deterioration of the solar battery caused
by the permeation of moisture is suppressed.

[0014] (b) A thin film, which is formed of fluororesin (refractive index:
1.34) with a thickness of about 10 nm to 20 nm and which functions as a
protective member or a water repellent film, may be formed on the side
surface of the homogenizer.

[0015] Since the sealing resin or the transparent resin for protecting a
solar battery cell is exposed to the severe environment of the
concentrator photovoltaic device, heat resistance and weather resistance
are necessary. Materials containing silicon resin as a base material are
currently used for these purposes. Generally, silicon resin has good
weather resistance. However, since it is used in an area in contact with
a severe environment, it is not possible to ensure sufficient weather
resistance just through silicon resin. Therefore, a material obtained by
adding a filler (for example, a glass compound) for increasing the
weather resistance of silicon resin is generally used as the sealing
resin.

[0016] The refractive index of silicon resin containing glass compound is
about 1.5, which is close to the refractive index (about 1.6) of a
homogenizer. If the periphery of a homogenizer is covered with sealant
formed of a material with such a relatively high refractive index, the
critical angle of the total reflection of light of the portion becomes
larger than the critical angle of a portion which is not covered with the
sealant.

[0017] On the other hand, when the shape of the homogenizer is a
trapezoidal shape with a smaller sectional area at the solar battery cell
side, the incidence angle of light (the angle between the normal line
direction of the reflection surface and the light incidence direction)
becomes small whenever reflection is repeated. Accordingly, if the lower
side surface of the homogenizer is sealed with a high refractive index
material, the probability that the incidence angle will become equal to
or smaller than the critical angle (that is, the probability that light
will leak) is higher near the lower side surface of the homogenizer.

[0018] In order to solve this problem, using a material with a relatively
low refractive index as the sealant may be considered. However, there is
no known material which has a low refractive index and is excellent in
heat resistance and weather resistance.

RELATED ART DOCUMENT

Patent Document

[0019][Patent Document 1] JP-A-2007-201109

[0020][Patent Document 2] JP-A-2006-313809

SUMMARY

[0021] In order to solve the problem described above, it is an object of
the present invention to provide a concentrator photovoltaic device with
both high conversion efficiency and high weather resistance.

[0022] According to an aspect of the present invention, there is provided
a concentrator photovoltaic device comprising: an optical concentrator
for concentrating sunlight; a solar battery cell; a homogenizer which is
provided above the solar battery cell such that a bottom surface thereof
faces the solar battery cell and which guides sunlight concentrated by
the optical concentrator to the solar battery cell; a sealant which
covers a side surface of a lower portion of the homogenizer and the solar
battery cell; and a light transmission preventing layer provided between
the sealant and the homogenizer, wherein the homogenizer has a
trapezoidal shape in which a sectional area at the optical concentrator
side is larger than a sectional area at the solar battery cell side, a
relationship of nh>nf>nt is satisfied among a
refractive index nh of the homogenizer, a refractive index nf
of the sealant, and a refractive index nt of the light transmission
preventing layer, a thickness (H) of the light transmission preventing
layer is equal to or larger than 0.1 mm and equal to or smaller than 1.2
mm, and a relationship of 0.5≦b/a<1.0 is satisfied between a
height (b) of the light transmission preventing layer and a height (a) of
the sealant at a position where the light transmission preventing layer
is formed.

[0023] If a high refractive index material is used as the sealant which
covers the lower side surface of a trapezoidal homogenizer and a solar
battery cell, high weather resistance is obtained. In addition, if a
light transmission preventing layer with predetermined refractive index
nt, thickness H, and height b is provided between the sealant and
the homogenizer, leakage of light from the lower side surface of the
homogenizer can be suppressed. As a result, conversion efficiency can be
improved without reducing weather resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Illustrative aspects of the invention will be described in detail
with reference to the following figures wherein:

[0025] FIG. 1 is a schematic sectional view showing a concentrator
photovoltaic device according to an embodiment of the present invention;

[0026]FIG. 2 is a view showing the relationship between the number of
times of measurement of a solar battery cell and a relative short-circuit
current;

[0027] FIG. 3 is a view showing the relationship between the thickness H
of a light transmission preventing layer and the amount of relative
increase in a short-circuit current; and

[0028]FIG. 4 is a view showing the relationship between the ratio b/a of
the height "b" of the light transmission preventing layer to the height
"a" of the sealant and the amount of relative increase in a short-circuit
current.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION

[0029] Hereinafter, an embodiment of the present invention will be
described in detail.

[1. Concentrator Photovoltaic Device]

[0030] FIG. 1 is a schematic sectional view showing a concentrator
photovoltaic device according to an embodiment of the present invention.
In FIG. 1, a solar power generator 10 includes an optical concentrator
12, a solar battery cell 14, a homogenizer 16, sealant 18, and a light
transmission preventing layer 20.

[1.1. Optical Concentrator]

[0031] The optical concentrator 12 is (the primary optical system) for
concentrating sunlight and emitting the concentrated sunlight to the
solar battery cell 14. A method using a concentrating lens, such as a
Fresnel lens, and a method using a concentrating reflector, such as a
concave mirror, are known as concentrating methods. In the present
invention, any method may be used.

[0032] In the example shown in FIG. 1, the optical concentrator 12 is
formed by a concentrating lens. The concentrating lens is advantageous in
that it is secure against dust or dirt, excellent in durability, easy in
heat radiation, and the like. The optical concentrator 12 is fixed above
the solar battery cell 14 using a support means (not shown).

[1.2. Solar Battery Cell]

[0033] The solar battery cell 14 is a cell for converting emitted light
into electric power. In the present invention, neither the structure of
the solar battery cell 14 nor the material used to form it is
particularly limited, and it is possible to use cells with various
structures and materials.

[0034] Generally, a solar battery cell has a structure in which a bottom
electrode, a semiconductor layer showing a photovoltaic effect, and an
upper electrode are laminated in that order. An antireflection film may
be formed on the surface of the semiconductor layer. As materials for the
semiconductor layer, for example, crystalline silicon and group III-V
compound semiconductor represented as InGaP/InGaAs/Ge are known.

[0035] A solar battery cell is generally fixed on a substrate, and various
kinds of components required for power generation of the solar battery
cell are provided on the substrate. In the example shown in FIG. 1, an
insulation layer 24 and a plate 26 are formed on a substrate 22 in that
order, and the solar battery cell 14 is fixed on the plate 26 with a lead
electrode 28 interposed therebetween.

[0036] The substrate 22 serves to support the components, such as the
solar battery cell 14. The material for the substrate 22 is not
particularly limited, and various materials may be used. Examples of the
material for the substrate 22 include aluminum, copper, and the like. The
optical concentrator 12 is fixed to the substrate 22, or the substrate 22
is fixed to a fixed frame (not shown).

[0037] The insulation layer 24 serves to insulate one lead electrode 28,
which is connected to a bottom electrode (not shown) of the solar battery
cell 14, and the other lead electrode (not shown), which is connected to
an upper electrode (not shown) of the solar battery cell 14. Various
kinds of insulation materials may be used for the insulation layer 24.
Examples of the material for the insulation layer 24 include (a) resin
material in which glass fiber, alumina powder, and the like are
distributed (b) ceramics with high heat conductivity, such as alumina.

[0038] The plate 26 serves to radiate the heat of the solar battery cell
14 and to reinforce the solar battery cell 14. The plate 26 is provided
between the insulation layer 24 and the lead electrode 28. Various
materials with high heat conductivity may be used for the plate 26.
Examples of the material for the plate 26 include aluminum, copper, and
the like.

[1.3. Homogenizer]

[0039] The homogenizer (secondary optical system) 16 serves to guide
sunlight concentrated by the optical concentrator 12 to the solar battery
cell 14. In addition, the homogenizer 16 serves to homogenize light
energy by repeatedly performing total reflection of guided light on the
side surface. The homogenizer 16 is erected right above the solar battery
cell 14 such that the bottom surface faces the solar battery cell 14.

[0040] In the present invention, the homogenizer 16 has a trapezoidal
shape in which the sectional area at the side of the optical concentrator
12 is larger than that at the side of the solar battery cell 14. The
sectional shape of the homogenizer 16 is not particularly limited and may
be any of the circular, elliptical, and polygonal shapes. The angle of
the side surface of the homogenizer 16 (or the apex angle when the
homogenizer 16 is assumed to be a cone) is not particularly limited, and
various kinds of angles may be selected according to the purpose.

[0041] In the concentrator photovoltaic device 10, it is necessary to make
the solar battery cell 14 turn exactly in the direction of the sun all
the time in order to bend the sunlight with the optical concentrator 12.
For this reason, the concentrator photovoltaic device 10 generally
includes a tracking device for turning the solar battery cell 14 in the
direction of the sun. However, when the shape of the homogenizer is a
columnar shape, the conversion efficiency is significantly reduced if a
tracking error occurs. On the other hand, when the homogenizer 16 is made
to have a trapezoidal shape, there is an advantage in that conversion
efficiency is not reduced even if a slight tracking error occurs.

[0042] A material with a high light transmittance is used for the
homogenizer 16. Examples of the material of the homogenizer 16 include
(a) sodium containing glass, such as borosilicate glass and silicate
glass (b) aluminosilicate glass and sodium potassium barium glass.
Especially, the sodium containing glass is suitable as a material of the
homogenizer 16 because it is cheap and can be easily processed.

[0043] The refractive index nh of the homogenizer 16 needs to satisfy
predetermined conditions. These will be described later.

[0044] Various kinds of films may be formed around the homogenizer 16 when
necessary. For example, an antireflection film may be formed on the upper
surface (light incidence surface) of the homogenizer 16. Examples of the
antireflection film include (a) a TiO2/Al2O3
antireflection film with a multilayered structure of alumina and titania,
(b) an antireflection film including a magnesium fluoride layer or a
calcium fluoride layer.

[0045] In addition, a protective film for preventing the permeation of
moisture may be provided on the interface between the homogenizer 16 and
the solar battery cell 14. In this case, the protective film may be
formed of a material different from the light transmission preventing
layer 20, which will be described later, or may be integrally formed on
the lower side surface and bottom surface of the homogenizer 16 using the
same material as the light transmission preventing layer 20 as shown in
FIG. 1. In the case of forming the light transmission preventing layer 20
only on the lower side surface of the homogenizer 16, it is preferable to
provide a protective film on the bottom surface of the homogenizer 16.

[0046] It is preferable to use a material, which is high in light
transmission and heat resistance, for the protective film. In the case of
forming a protective film separately from the light transmission
preventing layer 20, examples of the material of the protective film
include gel type silicon resin, an acrylic resin film, and the like.

[1.4. Sealant]

[0047] The sealant 18 is for covering the lower side surface of the
homogenizer 16 and an exposed portion of the solar battery cell 14. Since
the sealant 18 needs to prevent the permeation of moisture into the solar
battery cell 14 for a long period of time, it is necessary to use a
material with high heat resistance and weather resistance. Examples of
the material of the sealant 18 include (a) silicon resin containing glass
powder, (b) self-adhesive RTV rubber filled with white and opaque
inorganic material powder which has high heat conductivity and high light
reflectivity (for example, calcium carbonate, titanium oxide, high-purity
alumina, high-purity magnesium oxide, beryllium oxide, and aluminum
nitride), (c) material obtained by adding 10% by weight or more of
fluorinated silicon resin to thematerial of (b), and (d) epoxy resin. The
refractive index nf and the height a of the sealant 18 need to
satisfy predetermined conditions. These will be described later.

[1.5. Light Transmission Preventing Layer]

[0048] The light transmission preventing layer 20 serves to prevent
transmission of light from the lower side surface of the homogenizer 16
and is provided between the sealant 18 and the homogenizer 16. The light
transmission preventing layer 20 may be formed only on the lower side
surface of the homogenizer 16. Alternatively, as shown in FIG. 1, the
light transmission preventing layer 20 may be integrally formed on the
lower side surface and the bottom surface of the homogenizer 16. That is,
the light transmission preventing layer 20 may also serve as the
protective film described above.

[0049] When the light transmission preventing layer 20 also serves as a
protective film, it is necessary to use a material which is high in heat
resistance and excellent in light transmission. On the other hand, when
the light transmission preventing layer 20 is formed only on the lower
side surface of the homogenizer 16, the light transmission preventing
layer 20 does not necessarily need to be formed of a light-transmissive
material. Examples of the light transmission preventing layer 20 include
silicon resin, fluororesin, and the like.

[0050] The refractive index nt, height b, and thickness H of the
light transmission preventing layer 20 need to satisfy predetermined
conditions. These will be described later.

[1.6. Refractive Index]

[0051] The various above-described materials are known as materials which
can prevent the permeation of moisture into the solar battery cell 14 and
have heat resistance and/or weather resistance. There is a correlation
between the weather resistance and the refractive index of the materials
described above. In general, a material with high weather resistance
tends to have a high refractive index. That is, a material which can be
used as the sealant of the solar battery cell 14 while simultaneously
satisfying the conditions of low refractive index and high weather
resistance is not known.

[0052] Therefore, in the present invention, a high refractive index
material with high weather resistance is used for the sealant 18 in order
to prevent the permeation of moisture into the solar battery cell 14. On
the other hand, the light transmission preventing layer 20 is provided
between the homogenizer 16 and the sealant 18 in order to prevent the
leakage of light from the lower side surface of the homogenizer 16, and a
low refractive index material which has low weather resistance but has a
low refractive index is used for the light transmission preventing layer
20.

[0053] That is, in the present invention, the relationship of
nh>nf>nt is satisfied among the refractive index
nh of the homogenizer 16, the refractive index nf of the
sealant 18, and the refractive index nt of the light transmission
preventing layer 20. This is a different point from a concentrator
photovoltaic device in the related art.

[0054] A specific combination of the homogenizer 16, the sealant 18, and
the light transmission preventing layer 20 is as follows

[0055] For example, when the homogenizer 16 is sodium containing glass
(refractive index: 1.6), it is preferable that (a) silicon resin
containing glass powder (refractive index: 1.5), acrylic resin
(refractive index: 1.5), polyester resin (refractive index: 1.5), and the
like, are used as the sealant 18 and (b) silicon resin (refractive index:
1.3 to 1.39), fluororesin (refractive index: 1.3 to 1.4), and the like
are used as the light transmission preventing layer 20.

[1.7. Thickness H]

[0056] The thickness H of the light transmission preventing layer 20
affects power generation efficiency and weather resistance. If the
thickness H of the light transmission preventing layer 20 is too small,
light easily leaks from the lower side surface of the homogenizer 16. In
order to suppress the leakage of light, the thickness H of the light
transmission preventing layer 20 needs to be equal to or larger than 0.1
mm. The thickness H of the light transmission preventing layer 20 is
preferably 0.2 mm or more, more preferably 0.3 mm or more, and still more
preferably 0.4 mm or more.

[0057] If the thickness H of the light transmission preventing layer 20
becomes large, the effect on power generation efficiency is eventually
saturated. In addition, if the thickness H of the light transmission
preventing layer 20 becomes too large, weather resistance is reduced. As
a result, moisture easily reaches the solar battery cell 14. Therefore,
the thickness H of the light transmission preventing layer 20 needs to be
equal to or smaller than 1.2 mm. The thickness H of the light
transmission preventing layer 20 is preferably 1.0 mm or less, and more
preferably 0.8 mm or less.

[1.8. Height Ratio b/a]

[0058] The ratio(=b/a) between the height "b" of the light transmission
preventing layer 20 and the height "a" of the sealant 18 at the position
where the light transmission preventing layer 20 is formed has an effect
on power generation efficiency and weather resistance. If the b/a ratio
is too small, light easily leaks from the lower side surface of the
homogenizer 16. Therefore, the b/a ratio needs to be equal to or larger
than 0.5. The b/a ratio is preferably 0.8 or more, more preferably 0.7 or
more, and still more preferably 0.6 or more.

[0059] On the other hand, if the b/a ratio is too large, weather
resistance is reduced. In addition, the refractive index of the light
transmission preventing layer 20 is larger than that of air. Accordingly,
when the height "b" of the light transmission preventing layer 20 exceeds
the height "a" of the sealant 18, the leakage of light from a portion
covered only by the light transmission preventing layer 20 is increased.
Moreover, since the light transmission preventing layer 20 is exposed to
the atmosphere, the weather resistance is reduced. Therefore, the b/a
ratio needs to be less than 1.0. The b/a ratio is preferably 0.98 or
less, and more preferably 0.96 or less.

[0060] Here, the "height b of the light transmission preventing layer 20"
refers to a distance from the bottom surface of the homogenizer 16 to the
upper end of the light transmission preventing layer 20.

[0061] In addition, the "height a of the sealant 18" refers to a distance
from the bottom surface of the homogenizer 16 to the upper end of the
sealant 18. When the height a of the sealant 18 is not fixed, it is
preferable that the conditions of the b/a ratio described above are
satisfied at least at the position where the light transmission
preventing layer 20 is formed.

[2. Method of Manufacturing the concentrator Photovoltaic Device]

[0062] When forming the light transmission preventing layer 20 on the
lower side surface of the homogenizer 16, a material of the light
transmission preventing layer 20 is first dissolved in an appropriate
solvent to obtain a solution. Then, the solution is applied on the lower
side surface of the homogenizer 16 (on the solar battery cell 14 side)
using methods such as immersion and brushing. In this case, the solution
may also be applied to the lower end surface of the homogenizer 16. By
removing the solvent after coating, the light transmission preventing
layer 20 can be formed at least on the lower side surface of the
homogenizer 16. Then, the lower end surface of the homogenizer 16 is
attached to the surface of the solar battery cell 14 fixed on the
substrate 22, and the lower side surface of the homogenizer 16 and the
solar battery cell 14 are sealed with the sealant 18.

[0063] For example, when the light transmission preventing layer 20 is
silicon resin, the lower side of the homogenizer 16 is immersed in a
solution of silicon resin for 1 to 5 seconds at room temperature. The
homogenizer 16 is pulled up from the solution and attached to the solar
battery cell 14 and is then cured (dried) at a temperature of 120 to
180° C. Through such a method, the light transmission preventing
layer 20 which also serves as a protective film can be formed on the
lower side surface and the bottom surface of the homogenizer 16.

[0064] Then, the concentrator photovoltaic device 10 according to the
present invention is obtained by fixing the optical concentrator 12 to
the substrate 22 or to the frame to which the substrate 22 is fixed.

[3. Operations of the Concentrator Photovoltaic Device]

[0065] In a state where the trapezoidal homogenizer 16 and the sealant 18
are directly bonded to each other, a part of light incident on the
homogenizer 16 leaks outside from the interface between the homogenizer
16 and the sealant 18. This reduces the power generation efficiency of
the solar battery cell 14.

[0066] This can be explained by the relationship between the refractive
index nh of the homogenizer 16, and the refractive index nf of
the sealant 18. That is, from Snell's law, the conditions of total
reflection of light can be expressed as sin θ=nf/nh.
Here, θ is an incidence angle of light (angle between the normal
line direction of the reflection surface and the light incidence
direction).

[0067] For example, when nh is 1.6, θ=69.6° if nf
is 1.5. Moreover, when nh is 1.6, θ=54.3° if nf is
1.3. That is, the probability of total reflection of light within the
homogenizer 16 becomes high, according to Snell's law, as the difference
increases between the refractive index nh of the homogenizer 16 and
the refractive index of a material (air or the sealant 18) in contact
with the homogenizer 16, and the power generation efficiency is improved
accordingly.

[0068] On the other hand, in order to suppress a reduction in the power
generation efficiency caused by a tracking error in a tracking type solar
power generator, it is necessary to make the homogenizer 16 have a
trapezoidal shape in which the area of the light incidence surface is
larger than the area of the surface facing the solar battery cell 14.
When the shape of the homogenizer 16 is a trapezoid, the incidence angle
θ of light becomes small as the total reflection of light within
the homogenizer 16 is repeated. If the apex angle of a trapezoid when the
homogenizer 16 is assumed to be a trapezoid is α and the incidence
angles at the time of k-th and (k+1)-th reflection of light are
θk and θk+1, respectively, there is a relationship
of θk+1=θk-α therebetween.

[0069] For this reason, if the lower side surface of the homogenizer 16 is
sealed with the sealant 18 formed of a high refractive index material,
the incidence angle θn of light near the sealant 18 becomes
smaller than the critical angle of total reflection of light. In this
case, light may leak to the side of the sealant 18. This reduces the
amount of light reaching the solar battery cell 14, resulting in a
decrease in the power generation efficiency. On the other hand, if a low
refractive index material is used as the sealant 18 in order to avoid
this, the weather resistance of the sealant 18 is reduced. As a result,
the solar battery cell 14 deteriorates easily.

[0070] In contrast, if a high refractive index material is used as the
sealant 18 which covers the lower side surface of the trapezoidal
homogenizer 16 and the solar battery cell 14, weather resistance is
improved. In addition, if the light transmission preventing layer 20 with
the predetermined refractive index nt, thickness H, and height b is
provided between the sealant 18 and the homogenizer 16, the critical
angle of total reflection of light near the sealant 18 becomes large and
the probability that light will leak from the lower side surface of the
homogenizer 16 is reduced accordingly. As a result, conversion efficiency
can be improved without reducing weather resistance.

EXAMPLES

First Example and First Comparative Example

[1. Manufacturing of a Sample]

[0071] The concentrator photovoltaic device 10 with the structure shown in
FIG. 1 was manufactured. Sodium containing glass whose refractive index
nh was 1.6 was used for the homogenizer 16, and the size (mm) of the
homogenizer 16 was set to quadrature11×quadrature7×L22.
In other words, the homogenizer 16 has an upper square surface and a
lower square surface, the length of each side of the upper square surface
is 11 mm, the length of each side of the lower square surface is 7 mm, a
length between the upper square surface and the lower square surface is
22 mm. Silicon resin containing glass powder whose refractive index
nf of was 1.5 was used for the sealant 18. Silicon resin whose
refractive index nt was 1.3 to 1.39 was used for the light
transmission preventing layer 20. The thickness H of the light
transmission preventing layer 20 was set to 0 mm (first comparative
example) or 1.0 mm (first embodiment), and the ratio (b/a) between the
height b of the light transmission preventing layer 20 and the height b
of the sealant 18 was set to 0 (first comparative example) or 0.9 (first
embodiment). The total number of solar battery cells 14 was set at 250.

[2. Test Method]

[2.1. Power Generation Efficiency]

[0072] Light of about 60 SUN was applied to the homogenizer 16, and a
short-circuit current at that time was measured for every solar battery
cell 14. A relative short-circuit current was calculated from the
acquired short-circuit current.

[0073] The relative "short-circuit current" refers to a ratio(=i/im0)
of a short-circuit current (i) of each solar battery cell to the average
value (im0) of the short-circuit current in the first comparative
example.

[2.2 Weather Resistance Test]

[0074] UV was emitted to the manufactured solar battery cell 14 using a UV
emitting device. Energy of UV emission was set to 400 mW/cm2, and
the emission time was set to 20 minutes. The UV emission was stopped
after the elapse of a predetermined time, and water was sprayed onto the
solar battery cell 14 for 3 minutes. Subsequently, such an operation was
repeated for 10 hours. After the end of the test, it was visually
determined whether or not there was a crack or stickiness in the sealant
around the homogenizer.

[3. Results]

[3.1. Power Generation Efficiency]

[0075]FIG. 2 shows the relationship between the number of times of
measurement of the solar battery cell 14 and a relative short-circuit
current. The "number of times of measurement of the solar battery cell
14" indicates the rankings when a total of 250 solar battery cells are
arrayed in order of high relative short-circuit current values. From FIG.
2, the following can be seen.

[0076] (1) The relative short-circuit current is different for each solar
battery cell 14.

[0077] (2) In a range where the number of times of measurement (ranking)
is 0 to about 200, the relative short-circuit current in the first
example is higher by about 2.5% than that in the first comparative
example.

[0078] Since the relative short-circuit current was correlated with the
power generation efficiency, it could be seen that the power generation
efficiency was improved when the light transmission preventing layer 20
was provided on the interface between the homogenizer 16 and the sealant
18.

[3.2 Weather Resistance]

[0079] Both the first embodiment and the first comparative example showed
good weather resistance without a crack or stickiness in the sealant.

Second Example

[1. Manufacturing of a Sample]

[0080] The concentrator photovoltaic device 10 was manufactured in the
same manner as in the first example except that the ratio b/a was set to
0.9 and the thickness H of the light transmission preventing layer 20 was
changed in a range of 0 to 1.2 mm. The total number of solar battery
cells 14 was 70 (7 levels×10).

[2. Test Method]

[2.1. Power Generation Efficiency]

[0081] Under the same conditions as in the first embodiment, a
short-circuit current at that time was measured for every solar battery
cell 14. The amount of relative increase in a short-circuit current was
calculated from the acquired short-circuit current.

[0082] The "amount of relative increase in a short-circuit current (%)"
refers to an increment(=(im2-im1)×100/im1) of the
average value (im2) of the short-circuit current of the solar
battery cell 14 of each level to the average value (im1) of the
short-circuit current of the solar battery cell 14 when b/a=0.9 and H=0.0
mm.

[2.2 Weather Resistance]

[0083] Weather resistance was evaluated under the same conditions as in
the first example.

[3. Results]

[3.1. Power Generation Efficiency]

[0084] FIG. 3 shows the relationship between the thickness H of the light
transmission preventing layer 20 and the amount of relative increase in a
short-circuit current when b/a=0.9.

[0085] From FIG. 3, the following can be seen.

[0086] (1) When H becomes 0.1 mm or more, the amount of relative increase
in a short-circuit current becomes 0.5% or more.

[0087] (2) When H becomes 0.1 mm or more, the amount of relative increase
in a short-circuit current is almost saturated.

[0088] (3) In order to obtain the amount of relative increase in a
short-circuit current which is equal to or larger than 1.0%, H needs to
be equal to or larger than 0.25 mm.

[0089] (4) In order to obtain the amount of relative increase in a
short-circuit current which is equal to or larger than 1.5%, H needs to
be equal to or larger than 0.42 mm.

[0090] (5) In order to obtain the amount of relative increase in a
short-circuit current which is equal to or larger than 2.0%, H needs to
be equal to or larger than 0.73 mm.

[3.2 Weather Resistance]

[0091] All of the solar battery cells showed good weather resistance
without a crack or stickiness in the sealant.

Third Embodiment

[1. Manufacturing of a Sample]

[0092] The concentrator photovoltaic device 10 was manufactured in the
same manner as in the first example except that the thickness H was set
to 1.0 mm and the ratio b/a of the light transmission preventing layer 20
was changed in a range of 0.1 to 1.2. The total number of solar battery
cells 14 was 120 (12 levels×10).

[2. Test Method]

[2.1. Power Generation Efficiency]

[0093] Under the same conditions as in the first embodiment, a
short-circuit current at that time was measured for every solar battery
cell 14. The amount of relative increase in a short-circuit current was
calculated from the acquired short-circuit current.

[0094] The "amount of relative increase in a short-circuit current (%)"
refers to an increment(=(im4-im3)×100/im3) of the
average value (im4) of the short-circuit current of the solar
battery cell 14 of each level to the average value (im3) of the
short-circuit current of the solar battery cell 14 when b/a=0.9 and H=1.0
mm.

[2.2 Weather Resistance]

[0095] Weather resistance was evaluated under the same conditions as in
the first embodiment.

[3. Results]

[3.1. Power Generation Efficiency]

[0096]FIG. 4 shows the relationship between the ratio b/a of the light
transmission preventing layer 20 and the amount of relative increase in a
short-circuit current when H=1.0 mm.

[0098] (1) When the ratio b/a becomes 0.5 or more, the amount of relative
increase in a short-circuit current becomes 0.45% or more.

[0099] (2) The amount of relative increase in a short-circuit current
becomes a maximum when the ratio b/a is 1.

[0100] (3) When the ratio b/a exceeds 1, the amount of relative increase
in a short-circuit current is reduced because a contact portion between
the homogenizer 16 and the air is reduced.

[0101] (4) In order to obtain the amount of relative increase in a
short-circuit current which is equal to or larger than 1.0%, the ratio
b/a needs to be equal to or larger than 0.75.

[0102] (5) In order to obtain the amount of relative increase in a
short-circuit current which is equal to or larger than 1.5%, the ratio
b/a needs to be equal to or larger than 0.85 and equal to or smaller than
1.16.

[0103] (6) In order to obtain the amount of relative increase in a
short-circuit current which is equal to or larger than 2.0%, the ratio
b/a needs to be equal to or larger than 0.95 and equal to or smaller than
1.05.

[3.2 Weather Resistance]

[0104] When the ratio b/a was equal to or larger than 1, a crack or
stickiness was found in the sealant. In contrast, when the ratio b/a was
less than 1, neither a crack nor stickiness was found.

[0105] While the embodiments of the present invention have been described
in detail, the present invention is not limited to any of the above
embodiments, and various modifications can be made without departing from
the scope and spirit of the present invention.

INDUSTRIAL APPLICABILITY

[0106] The concentrator photovoltaic device according to the present
invention can be used as a power generator for supplying electric power
to factories or homes.